The aim of this work is to determine the DNA sequence organization of class I genes contained in the swine major histocompatibility complex (SLA). Both genomic lambda and cosmid libraries have been constructed from a homozygous SLAdd animal. From these, porcine class I SLA genes have been isolated and characterized. Three genes have been sequenced in their entirety; three others are currently being sequenced. The organization of the class I SLA genes is similar to that of other class I genes, consisting of 8 exons encoding a leader polypeptide, three extracellular domains, a transmembrane and intracytoplasmic domain. Homologies between the SLA genes range from 85% between the two genes encoding the classical transplantation antigens to 60% between the most divergent genes. Although a variety of single nucleotide changes are observed in the exons of the genes, complex alterations are also seen, consistent with the interpretation that some mechanism of gene conversion may operate to generate polymorphism in this family.